Carbon product and method of making same



i aiented Oct. 26, 1937 UNITED STATES CARBON PRODUCT AND METHOD OF MAKINGSAME George E. Megow, South Milwaukee, Wis., as-

signor to Allen-Bradley Company, Milwaukee, Wis., a. corporation ofWisconsin No Drawing.

Application August 1, 1934,

Serial No. 737,927

2 Claims.

My invention relates in general to the obtainment of carbon in smallparticle size and more particularly to carbon of small particle sizeuniformly dispersed in a matrix.

Heretofore carbon in extremely small particle size or colloidal form wasobtainable in liquid suspension or in dry state which in these formswhen mixed with another substance did not give an even dispersion of thecarbon in the substance.

The manufacturers of electrical devices such as resistors used in radiocircuits, have sought for carbon in usable form of a small size. Theabove mentioned resistors are of necessity required to be of greatuniformity in physical characteristics and therefore an evendistribution of the carbon conductor throughout the body of the resistoris also required.

In a divisional application Serial Number 139,586, filed April 28, 1937,assigned to the Allen- Bradley Company, I have claimed the subjectmatter of my invention as it relates to a resistance unit and method ofmaking the same.

It is therefore an object of my invention to obtain carbon in smallparticle size and further to provide a method where this carbon of smallparticle size is uniformly distributed into the material of the articlein which the carbon is used.

I have found that electrical resistors as used 30 in radio circuits havebeen greatly improved by the use of carbon made according to myinvention. I have also found that carbon material made according to myinvention is successfully used in a coating for electrode elements inelec- 35 tronic devices. In this instance the coating is used to reducesecondary emission of the electrodes in the electronic devices.

It is a known fact that various gels such as a silica gel, andprecipitates such as aluminum hydroxide have the property of adsorbingtannic acid, gum catechu, Congo red, carbon suspensions and othersimilar carbon bearing materials.

I have discovered that this unique characteristic of gels andprecipitates can be utilized in preparing a finely divided form ofcarbon by first adsorbing the carbon bearing material, such as tannicacid, with aluminium hydroxide, and then carefully dehydrating andcalcining the combination to yield a finely divided form of carbon andthat this carbon is embedded in a matrix of aluminum oxide. In the caseof adsorption of tannic acid by silica gel and carrying out the sameprocess, the carbon is embedded in a matrix of silicon dioxide. I havealso used a combination of silica gel and aluminum hydroxide with tannicacid and in carrying out the same process the carbon is embedded in amatrix of silicon dioxide and aluminum oxide or a matrix of aluminumsilicate depending upon the calcining temperature used.

I have further discovered that if the process is carried out by firstpreparing the gel and then mixing the carbon bearing material with thegel, the amount of carbon bearing material adsorbed will be less thanthat which can be adsorbed by a modification of the process. To securethis increased adsorption it is only necessary to form the gel in thepresence of the carbon bearing material.

An example of this method is as follows: A five per cent solution, byweight, of aluminum chloride in water, is prepared, to which is added afifty per cent solution, by weight, of tannic acid in water, using threehundred fifty parts of aluminum chloride, dry basis, and two hundredfifty parts of dry tannic acid. When the solutions are mixed a slightprecipitate will form which later dissolves forming a clear brownsolution. The reactions are carried out in this dilute form so as toavoid the formation of aluminum tannate in an undissolved form beforeforming the precipitate.

The formation of the gel is accomplished by adding a base such asammonium hydroxide until the solution is neutral. As the precipitateforms the tannic acid is adsorbed by the aluminum hydroxide. The gel andadsorbed tannic acid is then separated from the excess tannic acid,water, and ammonium chloride by filtering and washing. In the mix givenabove practically all of the tannic acid will be adsorbed. At thepresent time I have not been able to determine the maximum amount oftannic acid that can be adsorbed by this process because the amountadsorbed depends upon the purity of the tannic acid, the dilution of thereagents, and temperature and rate of mixing during the formation of thegel. There is however a pronounced difference in the amounts of tannicacid adsorbed when the gel is prepared by the method just described ascompared with the method wherein the gel is prepared out of contact withthe tannic acid. The washed filter cake is carefully dehydrated and thencalcined to yield an extremely fine dispersion of carbon in a matrix ofaluminum oxide.

This method of preparation has the advantage of forming a gel in contactwith a carbon bearing compound in such a manner that a high degree ofuniformity of composition can be obtained. As the precipitate forms, adefinite amount of carbon bearing material is adsorbed and as there is aslight excess present all of the gel forms with the maximum amount ofcarbon bearing material uniformly distributed throughout the mass ofgell in the adsorbed state. Any excess not adsorbed by the gel isremoved by washing with water.

Minute wedge like chips of aluminum oxide and carbon have been examinedmicroscopically to .determine the fineness of the carbon and theuniformity of the carbon throughout the matrix. The thin edges that aretransparent when examined at sixteen hundred fifty diameters appear as aclear transparent light brown field in which it is impossible to locateparticles of car- 'bon of suflicient size to even estimate their size.

The dispersion is apparently quite uniform although it is diflicult tomake any exact statement because there are few, if any, regularly formedpieces which may be examined accurately.

This material can be used for various things in which finely dividedcarbon is essential. As stated previously two uses for this materialhave been found up to the present time and undoubtedly additionalapplications will be found.

One of the uses mentioned for this material is in the formation ofelectrical resistance devices wherein the finely divided carbon in aninsulating matrix forms a composite resistance. Material of this typecan also be used as an adedition to a ceramic mix or as an addition toan organic binder to form a resistor. In the manufacture of a ceramicunit a desirable procedure is to form the gel and carry out theadsorption process in a suspension of the ceramic forming body, followedby filtering, washing, forming, dehydrating, calcination and firing tovitrification.

This carbon material as for in electrodes in electronic devices is usedin the following manner. The formed material is ground with clay and anorganic binder to the cons stency of a smooth paint and applied as acoating to the electronic valve elements. The elements are then bakedand fired and the coating forms a thin film that has the property ofimproving the characteristics of the valve.

From the foregoing description it will be readily apparent to thoseskilled in the art to which an invention of the character describedappertains, that I provide a novel method of subdividing carbon intosmall particle size and provide for its use in an evenly dispersed form.I attribute the success of my invention to the even dspersion of carbonof small particle size in a matrix, however, I do not want to limitmyself to this disclosure as outlining the bounds of my invention butrather an example of what may be accomplished with the method I haveoutlined.

What I claim as my invention is:

1. The method of obtaining finely divided carbon embedded in aluminumoxide comprising the mixing of .tannic acid with aluminum chloride,adding ammonium hydroxide to form a gel and thereafter dehydrating andcalcining said gel.

2. The method of obtaining finely divided carbon embedded in aluminumoxide comprising the mixing of tannic acid with aluminum chloride,adding a base to form a gel, and thereafter dehydrating and calciningsaid gel.

GEORGE E. MEGOW.

